Method of operating a combusion installation and use of such a method for inhibiting vanadium corrosion

ABSTRACT

A method of operating a thermal installation other than a gas turbine and use of such a method for inhibiting vanadic corrosion is disclosed herein. Embodiments of the invention relate to a method of operating a thermal installation comprising a combustion chamber fed with a fuel contaminated with vanadium, with sulfur and possibly with sodium. The combustion chamber is also fed with boron and with magnesium, in quantities such that the magnesium molar ratio m=MgO/V2O5 and the boron molar ratio b=B2O3/V2O5 satisfy the conditions (i) m≧2+b; (ii) m≦3+2b; (iii) b≧0.5 and (iv) b≦2, so that the combustion products comprise magnesium vanadate, mixed magnesium boron oxide and possibly sodium borate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a national stage application under 35 U.S.C. §371(c) prior-filed, co-pending PCT patent application serial number PCT/US11/54696, filed on Oct. 4, 2011, which claims priority to U.S. provisional patent application No. 61/389,570 filed on Oct. 4, 2010, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to the protection against vanadic corrosion of thermal installations that are operated differently from gas turbines and burn vanadium-contaminated liquid fuels. It relates in particular to a method of operating a thermal installation, for example a steam boiler, fed with this type of fuel.

From an economic standpoint, it is becoming increasingly advantageous to utilize, in energy applications, certain low-value petroleum fractions such as: very heavy crude oils, distillation residues (from atmospheric or vacuum distillation), by-products resulting from deep conversion of oils (high cycle oils and slurries deriving from FCC (fluid catalytic cracking) units and possibly certain heavy distillates.

For this purpose, such fuels may be burnt in various thermal installations such as: gas turbines, boilers, furnaces, diesel engines, etc., for the purpose of producing heat or steam, mechanical energy or electricity. However, the presence in these oil fractions of organo-vanadium compounds mainly in the form of vanadium porphyrins generates corrosion problems in metal alloys and ceramics that are used as structural materials or as surface coatings (protective layers or thermal barriers) in parts of these installations exposed to the combustion gas.

BRIEF SUMMARY OF THE INVENTION

A method of operating a combustion installation includes feeding to the combustion installation a fuel contaminated with vanadium and feeding to a combustion chamber of the combustion installation one or more boron compounds and one or more magnesium compounds is disclosed. The quantities added of the one or more boron compounds and the one or more magnesium compounds are such that the molar ratio of MgO equivalents added to the combustion chamber relative to V₂O₅ formed in the combustion chamber from the vanadium in the fuel, is molar ratio m, and the molar ratio of B₂O₃ equivalents added to the combustion chamber relative to V₂O₅ formed in the combustion chamber from the vanadium in the fuel is molar ratio b. The molar ratios b and m satisfy the following conditions: (i) m≧2+b and (ii) b≧0.5.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an embodiment of the present invention pertaining to magnesium, boron, and vanadium ratios.

FIG. 2 is a diagram showing a more specific embodiment of the present invention pertaining to magnesium, boron, and vanadium ratios.

DETAILED DESCRIPTION OF THE INVENTION

In the present application, the term “combustion installations” shall generally refer to thermal installations which are not gas turbines, and which utilize a combustible fuel, in an embodiment, vanadium containing heavy fuel oils, which operate with a flue gas temperature of 300° C. to 1050° C., as mentioned below, which is similar to the flue gas temperature of a boiler. These combustion installations, such as boilers, can operate at steam pressures ranging from 300 psig to 2650 psig or at any sub-range of pressure within this range, but more typically will operate in the 1200 psig to 1500 psig range or any sub-range of pressure within this range. The combustion installations, such as boilers, may or may not be used in the production of superheated steam, but the unit may contain a superheater in an embodiment. Operational flue gas temperatures in target systems range from 300° C.-1050° C. (and any sub-range of temperature within this range), such as 400° C. to 900° C., 400° C. to 650° C., 500° C. to 800° C., and 500° C. to 600° C. Typical boiler furnace temperatures will be used as examples for combustion installations; however, all the conclusions drawn in this document can be extended to any type of thermal equipment that has operation temperature levels similar to boilers, including industrial furnaces and diesel engines but at the exclusion of the gas turbines that feature particularly high temperature levels and require specific inhibition conditions.

Any range or ranges disclosed in this description are deemed to include and provide support for any sub-range within such range or ranges. Any range or ranges disclosed in this description are deemed to include and provide support for any point or points within those range or ranges.

As per the usage, the gas temperature or Tg of a combustion installation, such as a boiler, in a given heating zone refers to the temperature of the hot flue gas. “Tw” refers to the tube wall temperature in a given boiler heating zone and is below “Tg” due to the circulation of boiler water inside the tubes.

The corrosion described above is called “vanadium corrosion”, and is due to the formation, in the flames, of vanadium compounds of oxidation state 5 that are distinguished by low melting points Tm that lie below the tube wall temperature Tw in some boiler zones, such as vanadium pentoxide (V2O5: Tm=675° C.) or compounds involving alkali metals, such as alkali metal metavanadates (NaVO3: Tm=628° C.; KVO3: Tm=517° C.; the eutectic of these two salts: Tm=475° C.) and V2O5/Na2SO4 mixtures (eutectic at 40 mol % Na2SO4: Tm=500° C.). Thus, it should be noted that the association of alkali metals (Na, K) with V2O5 is particularly deleterious because of the formation of compounds that are even more fusible and moreover more fluid and more conducting in the molten state. These compounds are transported in liquid form by the flue gases and may deposit on the tubing in the furnace (waterwall tubes) or may travel into the convection pass and deposit on the superheater, reheater, boiler or economizer tubes of the installation. The fraction that are deposited on the water wall tubes, and superheater, boiler and economizer areas may result in vigorous electrochemical attack characteristic of the molten electrolytic media associated with oxidizing agents, in this case vanadium at the oxidation state 5 itself, sulfur at the oxidation state 6 (SO₃ and sulphates) coming from the fuel, and any residual oxygen contained in the combustion gases. Vanadium corrosion may be inhibited by chemically trapping V2O5 within refractory and chemically stable compounds that eliminate the molten electrolytic medium and ipso facto this form of high temperature corrosion. Possible vanadium inhibitors are compounds based on alkaline-earth metals, such as calcium and magnesium salts. These inhibitors, injected into the furnace of the combustion installations to be protected, react with the vanadium compounds to form orthovanadates, pyrovanadates, and metavanadates, the melting points of which are above the melting points of V2O5 and of the corresponding sodium vanadate species. The orthovanadate may be written in the form M3(VO4)2 in which M denotes an alkaline-earth metal. In the particular case of magnesium, the magnesium orthovanadate (OV) having a melting point of 1074° C. is formed according to the following reaction:

V2O5+3MgO→Mg₃(VO₄)2.   (1a)

The pyrovanadate may be written in the form M2(V₂O₇) in which M denotes an alkaline-earth metal. In the particular case of magnesium, the magnesium pyrovanadate (PV) having a melting point of 980° C. is formed according to the following reaction:

V2O5+2MgO→Mg2V2O7.   (1b )

The metavanadate may be written in the form M(VO3)2 in which M denotes an alkaline-earth metal. In the particular case of magnesium, the magnesium metavanadate (MV) having a melting point of 742° C. is formed according to the following reaction:

V2O5+MgO→Mg(VO3)2.   (1c)

Reactions (1a), (1b), and (1c) are written with Mg as the metal M described above. However, reactions (1a), (1b), and (1c) can also be written with calcium or nickel instead of magnesium, and calcium oxide and nickel oxide can be used instead of MgO in embodiments of the present invention. In the present disclosure, Mg will be used as an example, but other alkaline-earth metals, such as calcum or barium, can take its place.

The molar ratios (MO/V2O5) corresponding to the orthovanadates, pyrovanadates and metavanadates are 3, 2 and 1 respectively.

This mode of inhibition, which consists in removing the corrosive vanadium derivatives from the exposed surfaces and in trapping them in reputedly stable refractory compounds, enables all materials, whether metallic, ceramic or composite, to be effectively protected. The inhibitor must be injected in sufficient quantity so as, on the one hand, to trap all the vanadium introduced by the fuel and, on the other hand, to form the desired vanadate species based on furnace operating conditions. In combustion installations, such as steam boiler systems, the ortho and pyro vanadate species have a sufficienty high melting point to fulfill the wanted protection. As far as the metavanadate form is concerned, it must be noted that the ash layers deposited on the walls are exposed to a temperature (“Td”) comprised between the skin temperature (“Tw”) of the metal and the flue gas temperature “Tg”. Therefore, the melting point (742° C.) of the metavanadate that is higher than Tw suffices to avoid the formation of fusible, corrosive slag upon the metallic wall, making magnesium metavanadate a suitable inhibition product as well; however since Tg can exceed 742° C. (e.g. in the furnace or in the superheater sections), any ash layer that would contain a substantial amount of Mg(VO₃)₃ may experience partial fusion and sintering of its outer portion, leading to deposits that are not easy to remove.

In practice, the (Mg/V) ratio targeted for the inhibition of vanadic corrosion in combustion installations, such as boilers, is 1 by weight, which corresponds to an atomic (Mg/V) ratio of 2.09 and to a molar (MgO/V₂O₅) ratio of approximately 4.2. Such dosage of MgO should enable, in theory, the formation of the very high melting point orthovanadate that requires a molar (MgO/V₂O₅) ratio of 3, according to reaction (1a).

All the inhibition methods have the common drawback of not reducing but, on the contrary, increasing the volume of ash that leaves the flame. The “magnesium-vanadium” ash formed during inhibition by MgO partly deposits on furnace wall tubes, superheater, reheater, and boiler tubes, with the result that the treatment increases the overall ash loading of the system.

The “ash deposition rate” may be defined as the ratio of the mass of ash deposited in the furnace to the mass entering the furnace over a given duration. This “ash deposition rate”, from which the rate of fouling and the impact on performance of combustion installations (such as boilers) directly result, is a complex parameter ascertainable only by experimentation since, besides the temperature and the velocity of the gas stream and of the ash particles suspended therein, depends on many other factors difficult to determine, such as: the chemical nature of the ash (based on the composition of the fuel); the ash particle size distribution; the angle of impact of the particles relative to the substrate; the state of the substrate (roughness; oxidation state); the hardness of the particles compared with that of the impact surfaces (these impact surfaces are initially the bare metal walls of the combustion installation, or a surface coating thereon, or the layers that form progressively thereon). The physical properties of these layers are themselves liable to change as a result of a physical transformation (compaction or densification) or chemical transformation of the ash. In particular, ash particles are also subjected to “sintering”. The latter phenomenon, which is essential in the aging process of any deposit, affects any crystalline solid heated to high temperature over long durations: the solid tends to densify by reduction in its porosity, to recrystallize and to harden. Consequently, irrespective of the method of inhibition, the long residence time of the ash on hot components is liable to result in the deposits being progressively sintered and becoming potentially more difficult to remove. Furthermore, the sintering is accelerated in the presence of a molten phase, which accelerates internal atomic diffusion.

To mitigate the losses in performance caused by excessively degraded operation of the boiler, it is essential for these deposits to be periodically removed from the deposit prone areas. In combustion installations, such as boiler systems, cleaning is performed both via off-line and on-line procedures. The online methodology is performed in several different ways. The principal methodology is through the use of soot blowing. Soot blowing is performed by blowing jets of air, steam or sometimes water (free of corrosive salts) onto the deposits to aid in removal. Soot blowers are positioned in locations that are prone to deposit accumulation and are controlled either manually or through an automated regimen. In some cases, soot blowing is supplemented via a manual lance that can be used to clear paths that are out of the reach of soot blowers

Where routine soot blowing is ineffective, a “chill and blow” can be performed. This process entails a reduction or cessation in fuel feed that results in lower firing and reduced system temperatures. The reduced temperature helps solidify the liquid deposits, if any, and destabilizes the deposit layer due to the differential expansion effect between ash and wall and improves overall removal efficacy of soot blowing.

Both methods combine a mechanical effect with the potential to dissolve and carry away the ash deposits. The dissolution assumes that the deposits have a soluble phase (such as magnesium sulphate) in an amount sufficient to destabilize, during its dissolution, the entire deposited layer, which will then be carried and collected in the ash hopper.

The offline cleaning is performed during yearly maintenance shutdowns and when operational problems (including if dictated by excessive slag formation) require or permit. These methods can include an extensive water wash after the shut down and cooling of the combustion installation such as a boiler, or a variety of mechanical methods used to physically remove the accumulated slag and deposit from the furnace.

Since both online and offline methods result in loss of steam generation or reduced operating capacity, these methods of restoring performance can have a substantial impact on system availability and on production.

Inhibitors based on alkaline-earth metals (e.g., magnesium and calcium) are very effective in protecting against vanadium corrosion. However, the very low solubility in water of calcium sulphate (Ca504) and the hardness and strong adhesion of the deposits that it forms make the above-mentioned cleaning methods more difficult. Calcium derivatives therefore are less likely to be used in practice as vanadium inhibitors.

Magnesium inhibitors, which are commercial additives very widely used, have three main drawbacks in boiler applications. p The first drawback stems from the fact that magnesium based ash intrinsically results in a high “ash deposition rate” on the hot parts and therefore results in particularly rapid fouling of the hot parts. This is a characteristic of magnesium sulphate-magnesium vanadate systems, which can be confirmed by simulation tests carried out in a “burner rig” but, as indicated above, cannot be deduced from a purely theoretical approach.

The second drawback is due to the limited thermal stability of MgSO4, since at high temperature a sulphation/desulphation equilibrium according to equation (3) is established:

MgSO4→MgO+SO3.   (3)

With increasing temperature, this equilibrium is shifted in the endothermic direction, i.e. to the right, and MgSO4 therefore tends to be desulphated. On water tubes, this effect essentially affects the outer portion of the deposits due to the existence of the already mentioned temperature gradient (T_(g)−T_(w)). Thus, the outer layer of the deposit faces the radiative heating of the furnace while the inner portion is cooled by the steam flowing inside the tube. Since MgO has a higher density than MgSO4 (3600 kg/m³ as opposed to 2600 kg/m³), there is also a physical contraction and an agglomeration of the deposit. Hence, the latter loses its porosity and becomes more difficult to dissolve or to be mechanically disintegrated, while its tendency to be sintered is increased by this densification. In addition, since it is the outer portion of the deposits that is affected, the formation of such refractory, impervious layer of MgO acting as a physical barrier decreases the efficiency of steam or water used as cleaning media since they cannot reach the core of the deposits. The water-soluble magnesium sulphate is thus replaced with magnesium oxide, which is neither soluble in water nor in all of the reagents compatible with the integrity of boiler materials.

The third drawback of inhibition using MgO lies in the sensitivity of magnesium-vanadium ash to sodium. Any traces of sodium contained in the fuel (fuels of combustion installations, such as boilers, are not routinely pre-washed for sodium removal) or in the air (Na2SO4 dust in an industrial environment or NaCl-rich fogs in a marine environment) are converted to Na2SO4 in the flames and are incorporated into the magnesium ash, either in the form of mixed sulphate Na6Mg(SO4)4 which melts at 670° C. or, owing to the strong affinity existing between sodium and vanadium, in the form of mixed vanadate NaMg4(VO4)3, which melts at 570° C. These two compounds not only make the ash more fusible and aggravate the sintering phenomenon, but are also potentially corrosive.

Because of this “parasitic” sodium and MgSO4 desulphation effects, inhibition using magnesium appears in fact to be a relatively complex process, the actual balance of which is not simply that of reactions (1a) to (1c) but involves the extensive chemistry of the (magnesium oxide—sodium/magnesium vanadates and sodium/magnesium sulphates) system, leading to the need for substantial overdosages of magnesium with respect to the minimum theoretical requirement. This explains why, in practice, although the typical (Mg/V) ratio used for the inhibition of vanadic corrosion in boilers is 1 by weight, corresponding to a molar (MgO/V₂O₅) ratio around 4.2, one does not form the orthovanadate Mg₃V₂O₈ (equation 1a) but mixtures containing in majority MgSO₄ and Mg₂V₂O7 (equations (1b)) with some minor contents of Mg₃V₂O₈, MgV₂O₆ (equations (1a) and (1c)) and MgO and trace amounts of the double Mg/Na vanadates and sulfates.

In view of the limitations of the current inhibition methods, it is therefore desirable to have an improved inhibition method meeting the following three objectives: (i) effectively trap the vanadium; (ii) deposit a minimum amount of ash, which can be removed, by an on-line method (such as soot blowing); and finally (iii) provide these two functions up to the highest possible limit temperature.

Now the Applicant has established that the association of boric oxide (B2O3) referred below to as a “second oxide”—with MgO (constituting the “first oxide”), enables the achievement of these objectives by strongly reducing the “parasitic” sodium and MgSO4 desulphation effects mentioned above.

Chemically, boric oxide B2O3 reacts rapidly and quantitatively when hot with MgO, to form, depending on the (Mg/B) atomic ratio, magnesium tetraborate MgB4O7 (“TB”), magnesium pyroborate Mg2B2O5 (“PB”) or magnesium orthoborate Mg3B2O6 (“OB”). These salts may also be written as MgO-2B2O3, 2MgO—B2O3 and 3MgO—B2O3, respectively, and they result from the following reactions:

MgO+2B2O3→MgB4O7: magnesium tetraborate (TB)   (4)

3MgO+B2O3→Mg3B2O6: magnesium orthoborate (OB)   (5)

2MgO+B2O3→Mg2B2O5: magnesium pyroborate. (PB)   (6)

Interestingly, B2O3 is also capable of reacting with magnesium sulphate MgSO4 with evolution of SO3:

MgSO4+2B2O3→MgB4O7+SO3   (4b)

3MgSO4+B2O3→Mg3B2O6+3SO3   (5b)

2MgSO4+B2O3→Mg2B2O5+2SO3.   (5b)

Vanadium pentoxide in turn reacts rapidly and quantitatively with the magnesium orthoborate and magnesium pyroborate to give magnesium vanadates:

3Mg3B2O6+V2O5→Mg3V2O8+3Mg2B2O5.   (6a)

4Mg2B2O5+3V2O5→3Mg2V2O7+2MgB4O7   (6b)

Magnesium orthoborate and pyroborate are therefore vanadium inhibitors according to reactions (6a) and (6b). The rapidity of all these reactions enables the composition of the ash in the combustion gases to be rapidly stabilized.

The use of B2O3 has five advantages.

The first advantage in using B2O3 as “second oxide” lies in the fact that, due to reactions (4b) to (6b) it considerably reduces the formation of MgSO₄ and prevents the secondary formation of the insoluble, fouling MgO by the desulphation process. Indeed, unlike MgSO4, magnesium orthoborate, pyroborate and tetraborate are thermally very stable and also have high melting points (1312° C., 1330° C. and 995° C. respectively) that substantially exceed the wall temperatures (Tw) encountered in combustion installations, such as boilers, making their use particularly advantageous and strongly preventing any sintering effect of the ash layer. The latter beneficial effect increases as the proportion of magnesium borate ash increases in the overall deposit.

The second advantage in using B2O3 lies in the low melting point (450° C.) of this oxide, which is close to that of V2O5. In the flame and immediately after the flame, the presence of an additional fraction of liquid represented by the molten B2O3, in addition to the molten V2O5, favors the reaction kinetics between the various species by accelerating the inter atomic diffusion, a well known effect often used in inorganic synthesis (e.g: the synthesis of the ferrite NiFe₂O₄ which is difficult when starting from the oxides is greatly facilitated when carried out using a mixture of molten nickel and iron nitrates). In particular, the formation of the desired magnesium vanadates find themselves accelerated by this kinetic effect. This is of considerable help in the case of combustion installations, such as boilers, where there are possible zones of lower temperature in which the reaction kinetics between MgO and V2O5 may become the limiting step of the inhibition process.

The third advantage in using B2O3 lies in the remarkable ash anti-deposition properties developed at high temperature by magnesium pyroborate and magnesium orthoborate, properties that have been discovered by the Applicant. These properties ensure particularly low ash deposition rates on hot components. Tests carried out on a burner rig over durations of 250 to 500 hours with typical dosages for boilers, as set out below, have shown, for example, deposition rates on average 4 times lower in the case of inhibition with boron than inhibition without boron (i.e. during an inhibition run performed with MgO alone).

The fourth advantage of magnesium-boron inhibitors lies in the very porous and friable texture of magnesium borate rich ash deposits which to a large extent explains the low deposition rate and makes it possible to remove these deposits using less aggressive physical methods including soot blowing, knowing that the mechanical entrainment effect of water and steam is also sufficient to remove them.

The fifth considerable advantage of magnesium-boron inhibitors, most particularly compared with MgO alone, lies in the fact that their performance is maintained in the presence of an appreciable amount of sodium. Specifically, when sodium is present in the fuel (or in the combustion air) and, after combustion, becomes incorporated into the ash in the form of Na2SO4, it may react with B2O3, even in the presence of magnesium, to form sodium borate Na4B2O5, which is not corrosive unlike the double sulphates and vanadates formed in MgO inhibition. This reaction may be written as:

2Na2SO4+3Mg2B2O5→Na4B2O5+2Mg3B2O6+2SO3.   (7)

Thus, it has been found that, in the operative conditions of combustion installations such as boilers, the protection by the magnesium-boron inhibitor remains effective for an (Na2SO4N2O5) molar ratio ranging up to 0.7 (i.e. an (Na/V) atomic ratio also ranging up to 0.7) and that the ash formed remains friable and non-adherent, despite a slight hardening. Moreover, the higher the magnesium borate content in the ash, the less this hardening effect is perceptible. This capability of neutralizing sodium and of eliminating its deleterious effects is a considerable advantage of magnesium-boron inhibitors.

The Applicant has identified the suitable dosages of magnesium and boron in combustion installations such as boilers. Herein, “b” is the molar ratio of boron (in B₂O₃ equivalents) to vanadium in the form of V₂O₅, such as the (B2O3N2O5) ratio and “m” is the molar ratio of magnesium (in MgO equivalents) to vanadium in the form of V₂O₅, such as the (MgO/V2O5) ratio. B₂O₃ equivalents include the following: precursors added to the combustion installation that form B₂O₃ in the combustion installation, B₂O₃ that is added in the form of B₂O₃, reaction products of B₂O₃, such as magnesium borates, that are added to the combustion installation, and precursors of such reaction products of B₂O₃. MgO equivalents include the following: precurors added to the combustion installation that form MgO in the combustion installation, MgO that is added in the form of MgO, and reaction products of MgO, such as magnesium borates, that are added to the combustion installation, and precursors of such reaction products of MgO. In other words, each B₂O₃ equivalent represents the addition, directly or indirectly, of one B₂O₃, and each MgO equivalent represents the addition, directly or indirectly, of one MgO. CaO equivalents are the same as MgO equivalents with a Ca instead of an Mg. If all of the magnesium is added in the form of MgO (each MgO being one MgO equivalent) and all of the boron is added in the form of B₂O₃ (each B₂O₃ being one B₂O₃ equivalent), then “b” is strictly the (B2O3/V2O5) molar ratio and “m” is strictly the (MgO/V2O5) molar ratio.

However, if the boron and the magnesium are added in other forms, such as Mg₃B₂O₆ (otherwise known as 3MgO—B₂O₃, which is three MgO equivalents and one B₂O₃ equivalent), or Mg₂B₂O₅ (otherwise known as 2MgO—B₂O₃, which is two MgO equivalents and one B₂O₃ equivalent) then “b” and “m” will incorporate this fact. Thus, in the case of the addition of one mole of B₂O₃ (one B₂O₃ equivalent) and one mole of MgO (one MgO equivalent) and one mole Mg₃B₂O₆(three MgO equivalents and one B₂O₃ equivalent) and one mole Mg₂B₂O₅(two MgO equivalents and one B₂O₃ equivalent), b is equal to 3/(V₂O₅), which is 3 B₂O₃ molar equivalents divided by the number of moles of V₂O₅ generated from the fuel. Similarly, m is equal to 6/(V₂O₅), which is 6 MgO molar equivalents divided by the number of moles of V₂O₅ generated from the fuel. In a (b,m) graph, the suitable (b,m) points represent the MgO and B2O3 equivalents, in an embodiment located in the zone defined by FIG. 1, as follows:

The representative point in an embodiment is above the straight line m=2+b to assure a good anti-corrosion protection, and b in an embodiment equals or exceeds 0.5 to obtain a substantial effect of boron.

This domain is therefore the upper, right angle delimited by the straight lines b=0.5 and m=2+b in the (b,m) plane and is referred below to as the “application domain”. The resulting ash may contain: magnesium pyrovanadate; magnesium orthovanadate; magnesium pyroborate and magnesium tetraborate, possibly magnesium orthoborate as a function of the position of the (b,M) point inside this zone and possibly sodium borate if sodium is present.

The Applicant has further identified a “preferential application domain” as being defined, in the (b,m) plane in FIG. 2, as follows:

-   m≧2+b (to assure a good anti-corrosion protection) -   b≧0.5 (to obtain a substantial effect of boron) -   m≦3+2b to avoid the formation of excess MgSO₄ as all the boron and     vanadate is already combined in orthoborate and orthovanadate     species respectively and cannot react any more with more MgO.

Finally, the “most preferential application domain” is represented by the trapeze at FIG. 2 defined by:

m≧2+b

b≧0.5

b≦1.5 to optimize the cost/effect ratio of boron

m≦5 to limit the cost of the magnesium component of the inhibition

In the present application, b can be between 0.25 and 2.5 (or any sub-range within this range). In an embodiment, b is between 0.5 and 1.5. Also, in the present application, m can be between 2.25 and 8 (or any sub-range within this range). In an embodiment , m is between 2.5 and 5. The present application envisions operating within any range or sub-range defined by the following ranges, and is deemed to provide support for operating in any point within the following ranges:

2+b≦m≦3+2b

2.25≦m≦8 (in an embodiment 2.5≦m≦5)

0.25≦b≦2.5 (in an embodiment 0.5≦b≦1.5)

With regard to the preparation of inhibitors based on MgO and B2O3, two methods of preparation are possible:

In the first method, the synthesis of the mixed MgO—B2O3 oxide or of precursors of the same occurs upstream of the combustion chamber of the installation. Starting from the chemical reactants, the mixed MgO—B2O3 oxide, or a precursor of this mixed oxide is synthesized and stored upstream of the combustion installation. The term “precursors” refers to a combination (or to a number of combinations) that contains magnesium and boron, which is not necessarily a defined chemical compound of magnesium and boron and which produces the desired mixed MgO—B2O3 oxide in the flames. Such precursors are for example sol-gels or other nanoscale structures. The mixed MgO—B2O3 oxide or its precursors, prepared in this way and stored, is injected in a suitable quantity either directly into the combustion chamber or at a point in the fuel circuit where it is intimately mixed with the fuel using a static or dynamic mixer. To obtain optimum inhibition efficiency, the mixed MgO—B2O3 oxide may be in nanoscale form. It may especially be advantageous to synthesize mixed MgO—B2O3 oxides or precursors in the form of either oil soluble substances or very finely divided particles or, in an embodiment, in the form of nanostructured substances.

In the second method, the synthesis of the mixed MgO—B2O3 oxide is carried out in situ, i.e. directly in the combustion chamber of the combustion installation, by reaction of two reactants introduced upstream of said combustion chamber (e.g. an aqueous solution of MgSO₄ and a solution of B₂O₃ in diethylene glycol) or by the transformation in said combustion chamber of a precursor of the mixed MgO—B2O3 oxide (e.g oil soluble forms of magnesium and boron). It is noted that the precursors of MgO or the mixed MgO—B₂O₃ can be taken into account in the calculation of “b” and “m” above since they result in MgO and mixed MgO—B₂O₃ species in the combustion chamber. In this case, the chemical reactants or the precursors are stored upstream of the combustion chamber of the combustion installation and injected, in suitable proportions and quantities, either at a point in the fuel circuit where they will be intimately mixed with the fuel using a static or dynamic mixer, or directly into the combustion chamber. The concept of “suitable proportions” refers to the ratios of magnesium to B2O3, whereas the concept of “suitable quantities” refers to the Mg/V dosing ratio. An interesting particular case is the preparation of an oil soluble precursor, the starting point of which may be a magnesium derivative of the “overbased sulphonate” or “overbased carboxylate” type. Such overbased magnesium compound can be borated by introducing boric acid in suitable proportions and by stirring for several hours between 50 and 200° C. Alternatively, a suitable oil-soluble precursor can be obtained by adding an oil-soluble boron compound, such as an alkyl borate of generic formula (Alk)3B, for example ethyl borate (C2H5)3B, to the magnesium overbased compound.

According to an embodiment of the present invention, the method described above is used to inhibit the vanadic corrosion of the combustion installation possibly in the presence of sodium. According to an embodiment of the invention, the metallic, ceramic or composite materials of a combustion installation, for example a combustion installation such as a boiler burning a fuel contaminated with vanadium, which may or may not be associated with sodium, are protected from vanadium corrosion by introducing into or forming in the combustion chamber of said installation, an inhibitor formed by a mixture of magnesium borates, such that the representative point of the (b,m) pair lies inside the domains defined above.

The primary method of cleaning up the installation is dry cleaning by soot blowing, as described above, with water washing being a secondary way of cleaning up the installation.

To better illustrate the invention, several embodiments are described below.

EXAMPLE 1

A combustion installation, such as a boiler, has a furnace temperature of 570° C., and produces steam by burning an average of 2,000 kg/hr of a very degraded heavy fuel oil containing 200 ppm vanadium by mass (generating thus 3.926 mole/hour of V2O5). Such boiler is treated with an oil soluble inhibitor containing 15.5% by weight of magnesium and 3.45% by weight of boron. The injection flowrate of the inhibitor is 2.46 kg/hr, representing:

-   0.381 kg/hr or 15.67 mole/hr of magnesium; so: m=15.67/3.926=4.00 -   0.085 kg/hr or 7.87 mole/hr of boron i.e. 3.93 mole/hr of B2O3; so:     b=3.93/3.926=1.0. -   These dosage conditions satisfy the relation “m=2+2b” lying in the     “most preferential application domain”.

During an operation period of 1200 hours, the average thermal power of the boiler is potentially 19.59 MW thermal, meaning an average efficiency of about 86%. After this period, furnace wall tube slagging is solid and friable. Slagging and superheater fouling are controlled with routine soot blowing and a chill and blow procedure after six weeks.

COMPARATIVE EXAMPLE 1

The same boiler burning on average 2,100 kg/hr of the same heavy fuel oil containing on average 190 ppm of vanadium (generating thus 3.916 mole/hour of V2O5) is treated with an oil soluble inhibitor containing 20% by weight of magnesium and no boron. The injection flowrate of the inhibitor is 1.91 kg/hr, representing 0.382 kg/hr or 15.71 mole/hr of magnesium.

So, in this second inhibition treatment, one has: m=15.71/3.916=4.01 (corresponding to an injection rate of magnesium very close to the one in example 1) and b=0.

During an operation period of 1150 hours, the average production of the boiler is potentially 19.37 MW thermal, meaning an efficiency of about 81%. After this period, slag is solid but difficult to remove with routine soot blowing. The accumulation of superheater ash deposits and furnace slag requires a chill and blow procedure after less than four weeks.

A comparison of Example 1 and Comparative Example 1 shows a potential gain in Example 1 of 5% absolute (6.2% in relative) in the boiler efficiency on an average over 1200 hours of operation (50 days) which represents a saving of about 37 metric tons of fuel per year of continuous operation (i.e. about US$ 10,000) and the avoidance of 121 metric tons of CO₂ release into the atmosphere.

This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

What is claimed is:
 1. A method of operating a combustion installation comprising feeding to the combustion installation a fuel contaminated with vanadium and feeding to a combustion chamber of the combustion installation one or more boron compounds and one or more magnesium compounds, in quantities such that the molar ratio of MgO equivalents added to the combustion chamber relative to the moles of V₂O₅ formed in the combustion chamber from the vanadium in the fuel, is molar ratio m, and the molar ratio of B₂O₃ equivalents added to the combustion chamber relative to the moles of V₂O₅ formed in the combustion chamber from the vanadium in the fuel is molar ratio b, and wherein the molar ratios b and m satisfy the following conditions: (i) m≧2+b and (ii) b≧0.5.
 2. The method according to claim 1, wherein the molar ratios b and m satisfy the additional condition: m≦3+2b.
 3. The method according to claim 2, wherein the molar ratios b and m satisfy the additional conditions: (i) m≦5 and (ii) b≦1.5.
 4. The method according to claim 1, wherein at least some of the boron and magnesium fed to the combustion chamber is in the form of mixed magnesium boron oxide.
 5. The method according to claim 4, wherein the mixed magnesium boron oxide is in nanoscale form.
 6. The method according to claim 1, wherein at least some mixed magnesium-boron oxide is formed in the combustion chamber from at least one precursor introduced upstream of the combustion chamber.
 7. The method according to claim 6, wherein the mixed magnesium-boron oxide is in nanoscale form.
 8. The method according to claim 1, wherein the combustion installation is operated at a flue gas temperature of 300° C. to 1050° C.
 9. The method according to claim 1, wherein the combustion installation is operated at a pressure of 300 psig to 2650 psig.
 10. The method according to claim 1, wherein the combustion installation is operated at a flue gas temperature of 300° C. to 1050° C. and at a pressure of 300 psig to 2650 psig.
 11. The method according to claim 1, wherein the fuel fed to the combustion installation comprises sulfur and possibly sodium.
 12. The method according to claim 1, wherein the combustion installation is a steam boiler.
 13. The method according to claim 1, wherein the one or more boron compounds comprise at least one of B₂O₃, MgB₄O₇, Mg₃B₂O₆, and Mg₂B₂O₅, and the one or more magnesium compounds comprise at least one of MgO, MgB₄O₇, Mg₃B₂O₆, and Mg₂B₂O₅, and wherein the molar MgO equivalents and the molar B₂O₃ equivalents are based on the moles of at least one of MgO, B₂O₃, MgB₄O₇, Mg₃B₂O₆, and Mg₂B₂O₅ added to the combustion installation.
 14. The method according to claim 1, wherein the one or more boron compounds comprise at least one of B₂O₃, Mg₃B₂O₆, and Mg₂B₂O₅, and the one or more magnesium compounds comprise at least one of MgO, Mg₃B₂O₆, and Mg₂B₂O₅, and wherein the molar MgO equivalents and the molar B₂O₃ equivalents are based on the moles of at least one of MgO, B₂O₃, Mg₃B₂O₆, and Mg₂B₂O₅ added to the combustion installation.
 15. The method according to claim 1, wherein the one or more boron compounds comprise B₂O₃, and the one or more magnesium compounds comprise MgO, and wherein b is the molar ratio B₂O₃/V₂O₅ and wherein m is the molar ratio MgO/V₂O₅.
 16. The method according to claim 13, wherein molar ratios b and m satisfy the additional condition: m≦3+2b.
 17. The method according to claim 13, wherein molar ratios b and m satisfy the additional conditions: (i) m≦5 and (ii) b≦1.5.
 18. The method according to claim 15, wherein at least one of the B₂O₃ and the MgO are in nanoscale form. 